1. Field
Embodiments described herein relate generally to an MRI (magnetic resonance imaging) apparatus and an MRI method.
Additionally, embodiments described herein relate to technology to limit energy of RE (radio frequency) pulses transmitted by an MRI apparatus to an object.
2. Description of the Related Art
MRI is an imaging method which magnetically excites nuclear spin of an object (a patient) set in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation.
In MRI, an RF coil is used to transmit an RF pulse to an imaging region to cause nuclear magnetic resonance. The resonance frequency of the RF pulse is proportional to the intensity of the static magnetic field of the MRI apparatus. For example, in the case of a static magnetic field of 1.5 tesla, the resonance frequency is 63.8 MHz. The RF pulse at this frequency causes an increase in temperature of the body of the object. Therefore, from the viewpoint of safety, the International Electrotechnical Commission (IEC) standard or other standard prescribe the upper limit of the energy of the RF pulse transmitted to the object.
Specifically, energy of the RF pulse absorbed by 1 kg of living tissue is referred to as a specific absorption ratio (SAR). It is prescribed that an SAR value for an arbitrarily set period of 10 seconds and an SAR value for an arbitrarily set period of 6 minutes do not exceed a first upper limit and a second upper limit, respectively. The upper limit of the SAR value varies depending on the imaging region, such as the whole body and the head.
To satisfy the safety standard concerning SAR, according to a conventional technique disclosed in U.S. Pat. No. 6,426,623, a sum of the energy values of the RF pulses transmitted to the object is calculated for each of the preceding 1 second, the preceding 5 seconds and the preceding 10 seconds.
And the RF pulse generator is stopped if any of the following three cases occurs. A first case is that the sum for the preceding 1 second exceeds a first predetermined value. A second case is that the sum for the preceding 5 seconds exceeds a second predetermined value. A third case is that the sum for the preceding 10 seconds exceeds a third predetermined value.
The conventional measure for safety surely prevents the actual dose from exceeding the upper limit of SAR. As a result, imaging can be performed under a safer condition than is required when the SAR value actually has a margin with respect to the upper limit. When the SAR value has a margin with respect to the upper limit, imaging can be performed under a more suitable condition by increasing the irradiation intensity or the number of slices in accordance with the margin, for example. Thus, in order to perform imaging under more suitable condition, there is a demand for a technique of determining SAR with higher accuracy.